The long-term goal of this project is to provide an integrated psychophysical description of the human ability to sense chemical irritants via she trigeminal nerve. Guided by the view that the sensitivity to irritants derives from elements of the nociceptive and thermal senses present in all types of skin, the proposed research will investigate the extent to which this common neural basis influences chemosensory perception throughout the trigeminal system. This objective will be pursued by (1) measuring the sensitivity to irritants across the full diversity of trigeminal regions--the oral cavity, nasal cavity, face and eyes--on a large number of subjects, and inferring from those data the-contributions of local and systemic factors to individual differences; (2) measuring individual differences in sensitization and desensitization produced by two different irritants on two kinds of skin, thus determining whether these phenomena vary locally or systemically; (3) providing the first quantitative investigation of long-term capsaicin desensitization in the trigeminal region and determining if menthol, which was recently found to produce short-term desensitization, has similar long-term hyperalgesic effects; and (4) investigating the role of counterirritation, a form of masking intrinsic to the nociceptive system, on the perception of multiple and/or spatially complex irritant stimuli within the trigeminal field. As well as providing new information about trigeminal chemoreception and the extent to which it may be influenced by systemic (nociceptive) factors, the proposed research will employ a battery of new psychophysical procedures that could prove useful for assessing the incidence, and nature of apparent hypersensitivity syndromes such as contact dermatitis and multiple chemical sensitivities. In addition, studies of long-term desensitization and counterirritation will provide basic information about how these two phenomena may impact on applied issues ranging from the perception and liking of hot and spicy foods to the use of chemical irritants as topical analgesics.